In the early 24th century, matter replication was primarily used for objects and characters that would be in direct contact with the occupants which gave them an extreme sense of realism. Water, for example, would feel like actual water because on some level, it was, and it could create experiences like odors. This also enabled simple matter to exist outside of the holodeck for brief periods of time (such as snow) before they would lose cohesion without the support of the holodeck grid and revert back to energy.

This, however, used an extreme amount of power, and caused repetitive problems to occur in its safe usage. Some time in the mid 24th century, this was slowly phased out in favor of simpler 'true' holographic technology by focusing on the photons contained within micro force fields. This was not only safer and used less power, but had more varied usage and could be easily controlled with quicker reaction times. Some would argue this made it lose its appeal, but advances in the technology has made it as real as the matter replication method, which is still used for more complex, tactile objects, as well as food, odors, etc.

Holodeck walls can generate holographic images that appear to extend for an unlimited distance, seemingly much larger than its own dimensions. In doing so, however, the holodeck is aware only of its users; it does not recognize its own created objects. For example, if a person were to throw a holographic rock at the holodeck's walls, the rock would not be allowed to pass beyond the wall (if it were of replicated matter). (TNG: "Encounter at Farpoint") It does this by continuously adjusting the projections of the force fields and the use of a force field "treadmill".

Holodecks are equipped with safety protocols to prevent serious injury during their use, though these can be disengaged by the user when required. While active, a force field that is likely to cause a certain level of physical harm to a living humanoid collapses before making contact with them, allowing them to escape uninjured. When protocols have been deactivated holographic obstacles have the same effects on a person as the objects or instances they simulate; holographic bullets or a steep drop could be fatal in such a scenario. (TNG: "The Big Goodbye"; Star Trek: First Contact; VOY: "Extreme Risk")

How the security protocols are circumvented differs; in one instance, it required the voice authorization of two senior officers, (TNG: "Descent") while in others the authorization of the individual such as the ship's captain, or the person who started the program was enough. (VOY: "Extreme Risk") Safety protocols can also be unintentionally disabled due to software errors or physical damage to the holodeck's hardware system. The status of safety protocols can be reviewed by the computer upon the request of an operator. The use of a tricorder within the holodeck can be also be done to query the current safety protocol status.

Holodeck characters have been known to include a program element designed to hide anachronisms to the program's time period, such as uniforms and communicators, and prevent them from raising the character's ire and curiosity. (VOY: "Spirit Folk")

Holodeck matter can impersonate real matter at the molecular level. (VOY: "Phage") Molecule-sized magnetic bubbles replace molecules in full resolution holographic objects, which a computer can manipulate individually in three dimensions. However, the complexity of electron shell activity and atomic motions that determine biochemical activity in living creatures cannot be projected holographically. This prevents replicators from duplicating life and resurrecting the dead. Advances in computer technology may allow this, permitting a person to live forever in any chosen environment while interacting with real people and objects visiting the holodeck.

The computer may use large magnetic bubbles to simulate surfaces and textures rather than create an object at the molecular level. However, objects created within the holodeck would not exist beyond the holodeck itself, as they only exist as energy. (TNG: "The Big Goodbye") Since holodeck technology can be used with replicator technology, there are some instances where real objects are replicated within the holodeck and are used to interact with the holographic program and/or users; since these objects are real material composed of matter, they can leave the holodeck fully intact.

A holodeck can modify the appearance of persons within it.

The holoprogram depicting the final mission of the NX-class starship Enterprise NX-01 can project uniforms suitable to the participants' role over them.

A holodeck is able to superimpose an entirely different appearance over a participant. (TNG: "The Offspring")

Holograms can be augmented with force beams to simulate solid, tangible objects or with replicator technology to create actual solid matter such as foodstuffs. All food eaten on the holodeck are replications. No other type of simulation would survive outside of the holodeck.

A holodeck also has the ability to create holodecks within holodecks, and holodeck programs are able to be saved to a cube that can be inserted into special devices with information to "last a lifetime". (TNG: "Ship in a Bottle")

The energy matrix of a holodeck is incompatible with other ship systems. (VOY: "Parallax")

Even though holographically created characters, just like characters in a story book, are never self-aware and never know that they are not real, there have been a few rare instances in which that rule has not held true. During a Sherlock Holmes holodeck simulation in the late 2360s, Geordi La Forge and DoctorKatherine Pulaski argued that playing with Data was impossible and unfair to them as he had memorized all the Sherlock Holmes novels and could easily solve the cases. In order to level the playing field, La Forge requested that the holodeck create an opponent intelligent enough to defeat Data.

Even though La Forge meant Holmes, his request had specifically noted Data. As a result, the holodeck created a self-aware holographic character of James Moriarty who was not only fully aware of his own consciousness, but who subsequently argued that he had a right to exist and leave the holodeck to pursue his life as he wished. Another holographic writer - known as "Felix" - created the fully self-aware program of Vic Fontaine for the crew of Deep Space 9, Vic being completely aware of his holographic nature despite being a 1950s lounge singer, often offering the crew personal advice on relationship issues. (TNG: "Elementary, Dear Data", "Ship in a Bottle"; DS9: "His Way", "Badda-Bing, Badda-Bang", "It's Only a Paper Moon")

The bridge of the USS Enterprise – the user could select any of the five (at the time) bridges of the various Federation starships named Enterprise to view. Captain Montgomery Scott only wanted to see the original Enterprise bridge, "no bloody A, B, C, or D." (TNG: "Relics")

Natasha Yar's Living Will – designed by Lieutenant Natasha Yar to be played in the event she was killed, where she bid farewell to her comrades. The Enterprise bridge crew unfortunately had to watch the program when Yar was killed by Armus on Vagra II. (TNG: "Skin of Evil")

Lieutenant Barclay's various programs – these included a mock-up of Ten Forward, where the user could attack Commander Riker and Lt. Commander Geordi La Forge to "blow off some steam" as well as win the heart of Deanna Troi; a mock-up of Counselor Troi's office where the user could receive counseling from a hologram rather than the real Troi; a woodland setting where the user could duel with recreations of Captain Picard, Data, and La Forge in a sword-fight, complete with a recreation of Deanna Troi as "The Goddess of Empathy"; a mock-up of the Enterprise-D bridge, where the user could bid farewell to the bridge crew; the Einstein program where the user could debate mathematics and science with Albert Einstein; a synaptic interface where the user could control the main computer of the Enterprise with the power of their own thoughts (however, removal of the user by conventional means would result in death); and various other programs. (TNG: "Hollow Pursuits", "The Nth Degree")

In early episodes of TNG, the series' production staff had an unwritten rule that the floors in a holodeck simulation shouldn't go below the floor level of the holodeck's door. This made sense, as burrowing down to the deck below would probably be inadvisable on a starship. Subsequent story requirements and set designs eventually influenced producers to alter their "rule," deciding that at least one holodeck was a multi-story chamber. (text commentary, Star Trek Generations (Special Edition)DVD)

The appearance of the holodeck on TNG was affected by having limited finances. Production Designer Herman Zimmerman commented, "We were in a budget constraint that made us do a set that is a wireframe look." Zimmerman and other members of the design team that worked on TNG had a long-standing interest in demonstrating the machinery of the holodeck from inside the room, though this was not made possible until the advent of the Cardassian holosuite in Star Trek: Deep Space Nine. (The Deep Space Log Book: A Second Season Companion)

For its first appearance on Star Trek: Voyager in "The Cloud", the exterior of the holodeck was the same set piece as had previously been seen on TNG, right down to the octagonal door frame, although all had been repainted to match the color scheme for the new Voyager corridors. It did not receive a square door arch and updated door panels until its second appearance.

There are many discrepancies between episodes pertaining to the abilities and limits of holodeck technology. For example, in "Encounter at Farpoint", the young Wesley Crusher remains wet with holodeck water, after exiting into a corridor. In "Elementary, Dear Data", a piece of paper given to Data by James Moriarty is able to be carried outside of the holodeck and into a hall, but upon Moriarty's return in "Ship in a Bottle", a book thrown outside of the holodeck instantly disappears.

Also, in "The Big Goodbye", Cyrus Redblock and Felix Leech disappear slowly after a few moments outside of the holodeck, although a lipstick smudge from a holographic character stays with Picard all the way onto the bridge. Although these inconsistencies can be partially explained by the difference in the types of objects leaving the holodeck, it still leaves quite a few questions about what exactly constitutes the differences. The holodeck can use a degree of replication to make realistic objects for the holodeck occupant to use, so there is a possibility of the computer replicating a real piece of paper with the picture on, as it would be a relatively simple pattern.

Some may argue that another discrepancy is the need for holodeck users to change into the appropriate costumes before entering and leaving the holodeck, since the holodeck has the ability to change the appearance of its users (established in ENT: "These Are the Voyages..."). But this may just be an issue of taste, on the user's possible preference of replicated clothes versus holographic clothes. Another theory is that the settings of the Holodeck can be altered to not only make weapons be lethal as seen in Star Trek: First Contact where Picard kills the Borg in a ballroom suite with a machine gun but also to make what events transpired inside it real life like the aforementioned lipstick smudge on Picard